The newly discovered Upper Miocene to Upper Pliocene San Gregorio assemblage of the Purisima Formation in Central California has yielded a diverse collection of 34 marine vertebrate taxa, including eight sharks, two bony fish, three marine birds (described in a previous study), and 21 marine mammals. Pinnipeds include the walrus Dusignathus sp., cf. D. seftoni, the fur seal Callorhinus sp., cf. C. gilmorei, and indeterminate otariid bones. Baleen whales include dwarf mysticetes (Herpetocetus bramblei Whitmore & Barnes, 2008, Herpetocetus sp.), two right whales (cf. Eubalaena sp. 1, cf. Eubalaena sp. 2), at least three balaenopterids (“Balaenoptera” cortesi “var.” portisi Sacco, 1890, cf. Balaenoptera, Balaenopteridae gen. et sp. indet.) and a new species of rorqual (Balaenoptera bertae n. sp.) that exhibits a number of derived features that place it within the genus Balaenoptera. This new species of Balaenoptera is relatively small (estimated 61 cm bizygomatic width) and exhibits a comparatively narrow vertex, an obliquely (but precipitously) sloping frontal adjacent to vertex, anteriorly directed and short zygomatic processes, and squamosal creases. Fossil odontocetes include the lipotid “river dolphin” Parapontoporia sternbergi (Gregory & Kellogg, 1927), four true porpoises including a bizarre new genus also known from other strata (Phocoenidae indet., Phocoenidae unnamed genera 1 and 2, and cf. Phocoena), an indeterminate delphinid (Delphinidae indet.) a pilot whale-like delphinid (cf. Globicephalinae indet.), an undetermined sperm whale (cf. Physeteroidea indet.), and an indeterminate odontocete. The new record of Parapontoporia sternbergi is noteworthy as it represents the first association of any earbones (petrosal, tympanic bulla, malleus, and incus) for the extinct genus. Discovery and description of a complete marine mammal assemblage permits faunal comparisons with other published Pliocene marine mammal assemblages from around the globe. The aggregate Pliocene marine mammal assemblage from eastern North Pacific (ENP) shares little in common with the modern fauna, and is mostly composed of extinct genera; notably, phocoenids and odobenids were more diverse than in the ENP today. This indicates that the modern fauna of the ENP did not emerge until after the end of the Pliocene, and probably sometime during the Early Pleistocene. The Pliocene ENP assemblage is similar to that of Japan, and the North Pacific in general shares little with south Pacific, Mediterranean, or North Atlantic marine mammal assemblages, indicating the North Pacific hosted a provincial marine mammal fauna that evolved in isolation from the modern marine mammal fauna, which had already appeared in the North Atlantic by the Early Pliocene.